Transistor components, such as MOSFETs, are widely used as electronic switches in the industrial and automotive area. MOSFETs can be operated at high switching frequencies and compared to relays, have a small size.
However, losses occur when a MOSFET is operated. These losses mainly include ohmic losses and capacitive losses and occur as discussed now.
Although MOSFETs can be operated at high switching frequencies, they do not switch on or off abruptly, but they gradually change between an on-state, in which an ohmic resistance of the MOSFET assumes its minimum value, and an off-state, in which the MOSFET blocks and prevents a current flow. The minimum value of the ohmic resistance of an MOSFET is also known as on-resistance. Ohmic losses occur when the MOSFET is in its on-state and are due to the MOSFETs on-resistance. When the MOSFET changes its operation state from the on-state to the off-state, or vice versa, switching losses occur additionally during these transition phases. In many applications, the switching losses dominate the total MOSFET losses at low load currents.
Further, a MOSFET includes a voltage dependent output capacitance (usually referred to as COSS) which usually includes a drain-source capacitance CDS between its drain and source terminals and a gate-drain capacitance CGD between its gate and drain terminals. When the MOSFET transitions from the on-state to the off-state, the output capacitance is charged, i.e., energy is stored in the output capacitance; the output capacitance is discharged, when the MOSFET transitions from the off-state to the on-state. The output energy EOSS, which is the energy stored in the output capacitance, is mainly dependent on the voltage across the drain-source path when the MOSFET is in its off-state and is dependent on the capacitance value of the output capacitance. The energy stored in the output capacitance defines the capacitive losses of the MOSFET. In many applications, the capacitive losses dominate the switching losses under typical load conditions.
The ohmic losses are proportional to the square of the load current, while the capacitive losses do not depend on the load current. Therefore, dependent on the specific load conditions, the ohmic losses or the capacitive losses may prevail. For example, when a load connected to the MOSFET draws a low load current, so that a low current flows through the MOSFET in its on-state, the capacitive losses may mainly determine the overall losses. Whereas, when the load draws a high load current, the ohmic losses and switching losses during transition phases may mainly determine the overall losses. The switching losses during transition phases and the capacitive losses are directly proportional to the switching frequency of the device.
In addition, the output charge QOSS, which is the charge stored in the output capacitance, is important for some applications. For example, the turn off delay time of the MOSFET at low load currents is dominated by the output charge. This is the charge which has to be stored in the output capacitance before the transistor is completely turned off. This output charge is provided by the load current.
There is, therefore, a need to provide a circuit arrangement with a transistor component, such as a MOSFET, in which dependent on the load conditions the losses are minimized.